FIG. 1 illustrates a prior art brushless three phase alternating current power generating system 10 of the type manufactured by the Assignee of the present invention for applications in airframes. The basic power generating system 10 uses a permanent magnetic generator (PMG) 12 to produce electrical power for exciting a field winding 16 of an exciter 14. Three phase alternating current is produced by the exciter 14 from a three phase winding 18 which is wound on a rotor which is part of a rotating assembly 19 mounted on a single shaft. The rotating assembly 19 also contains a full wave rectifier 20 and a field winding 24, which is wound on a rotor 22 of a main generator, and optionally a permanent magnet rotor of the PMG 12. However, the permanent magnet rotor may be mounted on a second shaft driven with the rotors of the exciter 14 and main generator 22. The output of the three phase winding 18 is rectified by the three phase full wave rectifier 20 which drives the field winding 24 of the main generator 22. Three phase output current is produced by the three phase output winding 26 which is connected to a neutral N. A voltage regulator 28 is coupled to a point of reference 30 of the AC output of the main generator 22, to the exciter field winding 16 magnetically linked to the three phase winding 18 and to the PMG 12. The voltage regulator 28 pulse width modulates a current flowing through the field winding 16 as a function an error signal which is proportional to a difference between a voltage at the point of reference 30 and a reference voltage which is proportional to a desired voltage at the point of reference. The pulse width modulation of the current flowing in the field winding 16 is conventional. The aforementioned brushless power generating system 10 is conventional in airframes. The power for driving the rotor of the permanent magnetic generator 12, rotor of the exciter 14 on which is wound the three phase winding 18 and the rotor of the main generator on which is wound the field winding 24 is produced by a constant speed drive transmission coupled to an airframe propulsion engine.
A resistor 32 is coupled in parallel with the field winding 24 for the purpose of attenuating high frequency differential mode noise which is outputted by the three phase full wave rectifier 20 and is identified in FIG. 1 by I.sub.D. As a consequence of the inductance of the field winding 24, high frequency differential mode noise is shunted by the resistor 32 to avoid magnetically coupling the differential mode noise from the field winding 24 to the three phase output winding 26.
The use of a common mode inductor to attenuate common mode noise flowing to an electrical load is known. A common mode inductor has a pair of windings magnetically coupled to a magnetic core in phase opposition to each other in series with an electrical load. The common mode inductance is for the purpose of attenuating a small magnitude common mode noise flowing in both of the leads leading to the electrical load in the presence of a larger differential mode noise. An example of a common mode inductor is disclosed in U.S. Pat. No. 4,683,529.
Airframe manufacturers have specifications limiting the level of noise current as a function of frequency which may be present in electrical power used by the airframe. The limit 106 of FIG. 3 is representative of the noise current as a function of frequency which may be present on an airframe power bus. The limit 106 has a constant magnitude for frequencies above a frequency at the point of inflection 108. The hatched area of the current I.sub.C represents a portion of the output current from a main generator which exceeds the limit 106 which may be produced by prior art 400 Hz electrical power generating systems for airframes prior to filtering. The hatched portion of the output current above the limit 106 is a frequency range of approximately between 100 kHz to 5 MHz. Conventional filtering of the output current from the main generator is disadvantageous in that the inductors and capacitors of the filter must be sized to handle the high power output with the attendant weight, size and cost penalty.